RF plasma sources are widely used in plasma processing technology for large scale manufacturing of semiconductor chips (etching, deposition, ion implantation, sputtering), large panel displays, and ion sources. Inductively coupled plasma (ICP) sources as well as microwave plasma (MWP) sources have been more popular in a new generation of plasma reactors due to their ability to maintain high-density plasma at very low gas pressure, and due to their capability to separate a process of plasma generation from an ion acceleration.
One of the most advanced dense-plasma sources based on an inductive RF discharge (ICP) is shown in FIG. 2. This typical ICP source comprises a flat helix inductor coil 4 and a metallic discharge chamber 1 (filed with operating gas) having a quartz window 8 which separates a discharge volume of the chamber 1 from the inductor coil 4 thereby maintaining a plasma within the chamber. The direction of the electric field induced by the coil 4 is shown as a circular arrow in FIG. 2. The base 6 of the chamber contains either plasma processed substrate or an ion extracting arrangement for creating an ion beam. The external RF power source 5, connected to the inductor coil 4 via a matching device (matcher) 7, maintains the RF current in the inductor coil 4. This RF current induces an azimuthal RF electric field which maintains an azimuthal RF discharge current producing a plasma. The same configuration enables one to consider the ICP inductor as an electrical transformer where the inductor coil 4 is an actual primary winding and the plasma is a single closed turn of a virtual secondary winding. The matcher 7 is an essential part of the ICP inductor. It performs two important functions. First of them is to match the 50 Ohm conventional output resistance of the RF power source 5 with the inductor coil 4 impedance (depending on plasma parameters) for efficient power transfer to an ICP inductor. The second one is to tune the inductor coil 4 circuit to a resonance with an operating frequency, thereby, to enhance resonantly the RF current in the coil 4.
For a typical ICP inductor driven at a standard industrial application frequency of 13.56 MHZ, and with the RF power transferred into the plasma being around 1 kW, the inner volume of the discharge chamber 1 is a few liters, and the operating gas pressure is in the range 1-100 mTorr, the resonant RF current of the coil is a few tens of Amperes, and the RF voltage across the inductor coil 4 is a few kV. Under these conditions, the RF power loss in the matcher, connectors, and the inductor coil itself (due to its final resistance) is comparable to that transferred to the plasma. Moreover, due to the coil 4 and the metallic chamber 1 proximity, an essential RF current is induced along the chamber wall. This effect results in an additional power loss because of chamber heating. Therefore, a power transfer efficiency to the plasma is essentially less than 1, since a perceptible power has been dissipated in the ICP source hardware of the practically realized devices.
The large scale of RF voltage across the inductor coil 4 (a few kV) creates a considerable capacitive coupling between the coil and the plasma resulting in a capacitive current through the plasma to the chamber wall, and also in a high dc negative potential on the inner surface of the quartz window 8. The mentioned high dc potential accelerates the plasma ions toward the window causing window surface erosion and plasma contamination. Additionally, the capacitive RF current increases the plasma dc potential reference with respect to the chamber. This effect leads to a limitation (from the bottom) of minimum energy of the ions coming to a substrate 6 and is capable of damaging the substrate. Generally, the presence of high RF voltages on the coil 4 and the matching device 7 causes various serious problems (corona, sparking, breakdown) and costly efforts to prevent them.
The present invention enables one to overcome all the considered problems by insertion of a closed core with a high permeability (ferrite core) into an actual primary winding (inductor coil) and into a virtual secondary winding (induced plasma), instead of an air-core used for the conventional ICP inductor shown in FIG. 2.